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This spring, the mountains of the Pacific Northwest, Sierra Nevada, along with the Northern Rockies, were more than just snow-capped — they were buried in the white stuff. In fact, many locations still have more spring snowpack than has been seen in decades.

Head south across the 40th parallel, however, and things are dramatically different. While there is still above average snow in parts of southern California, a relatively snow-less winter and spring has left much of the Southwest in a drought that has fostered record wildfires, and already local officials are worried there won’t be enough water to get through the summer months ahead.

As of May 1, 2011, snowpack in much of the West was well above average, thanks to a cold and snowy winter and spring seasons. In the Southwest, however, snowpack was well below average. Credit: NRCS.

The current north-south contrast in Western spring snowpack is not unusual, though. It’s completely normal, and it’s exactly what climate scientists and meteorologists expect during years when the average weather pattern is influenced by La Niña, a climate cycle associated with cooler than average sea surface temperatures in the eastern tropical Pacific Ocean.

But while this north-south contrast turns out to be a regular feature of Western climate variability, new research published today in the journal Science shows that, when compared to typical snowfall during the past 1,000 years, the snowpack patterns appear to be changing as average temperatures climb in response to increasing amounts of greenhouse gases in the atmosphere. Scientists say that as temperatures continue to rise, this reduced snowpack will lead to more frequent water shortages in some increasingly thirsty Western states.

“We’re talking about simultaneous snowpack decline in Western watersheds that support 70 million people,” says U.S. Geological Survey (USGS) climatologist Julio Betancourt, who co-authored the study. “We have to be concerned.”

For several years, scientists have observed that spring snowpack all across the West — in both northern and southern areas — has been decreasing. But now, Betancourt and a group of other climate scientists have found that the recent decline in spring snowpack is unlike almost anything that has happened in the region during the past 1,000 years. As part of the study published in Science, USGS ecologist Greg Pederson investigated tree ring data to learn how snowpack in the West, on either side of the 40th parallel, changed from year-to-year and decade-to-decade. Looking back over nearly a millennium, he found that when the north had high snowpack, the south typically experienced low snowpack, and vice versa.

But Pederson and Betancourt noticed a change starting in the mid 1980’s. On average, the amount of snow on the ground at the beginning of April each year was decreasing everywhere across the West, coinciding with unusually warm spring temperatures in recent decades. This has important consequences for water supplies used for drinking water and agriculture, since many areas, including Southern California, are dependent upon spring runoff for much of their water supplies.

Spring snowpack in the West is an essential water resource, particularly in Southwestern states that are prone to summer drought, like California, Nevada, Arizona and New Mexico. Credit: Wolfgang Staudt/flickr.

Betancourt says that as average spring temperatures have risen, the snowpack has begun to melt earlier in the year (this is not the case this year, with cold weather locking in substantial amounts of snow in many Western mountain ranges). Moreover, he says that in many places, the warmer temperatures mean that more precipitation falls as rain than as snow. So while the actual amount of precipitation hasn’t changed dramatically, the amount of rainfall has.

“Snowpack is the water resource of the West,” says Greg McCabe, a USGS hydrology expert who was not directly involved in the new study, “and temperature has a dominant impact on the snowpack.” If warming temperatures in the West cause the snowpack to decrease even more, McCabe says, or if the timing of the melt each spring changes, then summer water shortages could happen a lot more often.

McCabe says the new research provides valuable evidence about how snowpack has changed historically, and also how unusual the recent patterns appear to be.

According to Pederson and Betancourt’s findings, the way snowpack has changed in the past 30 years isn’t entirely unprecedented. There were two brief periods in the mid 1300’s and early 1400’s when there was less snow built up throughout the West.

“At both those other times in the past, we know it was warm then too,” explains Betancourt.

Now, however, climate scientists are predicting that average temperatures in the West are going to keep rising this century, due in large part to the buildup of greenhouse gases in the atmosphere. Compared to the past, when snowfall returned to normal as temperatures dropped, researchers now expect that warmer weather could keep spring snowpack on the decline for years to come.

“What’s happening in the spring for snow is critically important [for water availability] compared to what is happening in the dead of winter,” says Betancourt.

So are these findings consistent with the weather this spring, which has brought frequent snowfall to many mountain areas, such as California’s Sierra Nevada mountain range, into late May?

“There isn’t anything that has happened this year that isn’t consistent with what our new findings are,” says Pederson. Even as average temperatures increase in the coming years, there will still be cool years where an above average snowpack builds up in parts of the West, particularly during La Niña years. “But these years will tend to be weather noise on top of the larger climate changes we will see over many years,” he says.